Alternators producing a.c. power must generally possess an output having a known, constant frequency. Much equipment utilizing that electricity depends upon an a.c. diet having a known, stable frequency. Producing that current of constant frequency imposes severe limitations on the operation of alternators and the engines that power them.
Generally, the output frequency of an alternator depends directly upon the speed at which its rotor turns. If the rotor has a single magnetic pole pair, then the output frequency equals the shaft speed in revolutions per unit time. Where the rotor has n magnetic pole pairs, the output has a frequency n times greater than the rotor speed.
However, maintaining the frequency of the rotor shaft speed usually requires the prime mover to also move at a constant speed. Typically, the prime mover, of course, represents an engine which converts chemical fuel into mechanical rotation. However, the engine operates at a reasonably high level of efficiency only when under an optimum load. Yet, reducing the output required of the alternator still requires the engine to operate at the fixed frequency. When operating under these conditions of reduced load, the engine, maintaining its high speed, suffers a drastic reduction in its efficiency.
With a reduced load, the engine can only maintain efficiency by operating at a lower speed. However, that would deleteriously and unacceptably change the alternator's output frequency.
To allow for the maintenance of engine efficiency under reduced loads, central power plants employ a bank of a large number of alternators, each with its own prime mover. Under conditions of reduced demand, the utility operates only a reduced number of the alternators and shuts down the remainder. Those that remain working continue to experience a load that allows them to operate with reasonable efficiency.
However, the concept of a bank of alternators requires a large central utility. Only this type of installation can avail itself of the notion of operating only a limited portion of its total available generating facilities. Installations with only limited requirements for electricity cannot avail itself of a large number of alternators, operating only those required to efficiently support the load.
Installations which can employ no more than a single alternator at most have attempted to vary the relative speeds between the prime mover and the alternator. Using a gear box, for example, allows the prime mover to operate at different speeds depending upon the load imposed. Changing the gearing ratio allows the alternator to run efficiently and produce the electricity at the required frequency. However, this operation imposes a severe burden on the gear box itself. The gears remain in constant operation and operate under a substantial load. As a result, the gear box regularly wears out at short intervals and require refurbishment or replacement at substantial expense.
The Roesel alternator has attempted to solve the problem by providing an alternator producing a constant frequency at different rotor speeds. The alternator, a description of which appears in Aviation Week and Space Technology of Feb. 26, 1973, employs a centrally positioned stator, surrounded by a cylindrical rotor. The latter has a layer of magnetizeable material on its inner surfaces. An exciter head on the stator, in effect, imprints magnetic poles on the layer of magnetizeable material on the cylindrical rotor. The exciter, in fact, creates a sufficient number of poles on the rotor so that the rate at which the fields from these poles pass through the stator coils create the desired a.c. frequency.
However, the Roesel alternator appears to suffer from two limitations. First, the generator would not appear to readily submit to a scaling-up process to increase the total electrical power production. The strength of the magnetic fields created and the necessity of spinning the cylindrical stator appears to impose a limitation on the maximum power efficiently created.
Second, the maximum efficiency for an alternator occurs where the number of pole pairs for the stator equals that for the rotor. A discussion of this concept appears in the paper "The Roesel Generator, A Unique Variable Speed-Constant Frequency Generator" by R. R. Ott, R. J. Barber, and J. F. Roesel, presented at the I.E.E.E. Applied Magnetics Workshop, Marquette University, June, 1975, which discusses it in terms of length of the magnetic pole. As the rotor speed increases or decreases, the exciter prints a lesser or greater number of poles on the rotor, respectively. Only at one particular stator speed do both the stator and rotor have the same number of poles. At any other speed, the alternator loses efficiency. Thus, the Roesel alternator still does not accomplish a highly efficient generation of constant frequency a.c. current with varying rotor speed.
Further, printing and removing of the poles involves magnetizing and demagnetizing the rotor's magnetic material. These operations entail substantial magnetic hysteresis energy losses and, thus, reduced efficiency.